The present invention is directed to the field of pressed and sintered powdered metal components. The present invention has particular applicability to pressed metal parts which require annular grooves, undercuts, internal cavities and the like.
In recent times, powder metallurgy (P/M) has become a viable alternative to traditional casting and machining techniques for fashioning metal components. In the P/M process, powdered metal is added to a mold and then compacted under very high pressures, typically between about 20-80 tons per square inch. The compacted part is ejected from the mold as a "green" part. The green parts are then sintered in a furnace operating at temperatures of typically 2000.degree.-2500.degree. F. The sintering process effectively welds together all of the individual powered metal grains into a solid mass of considerable mechanical strength. The P/M process can be generally used to make parts from any type of metal and sintering temperatures are primarily determined by the temperatures of fusion for each metal type.
P/M parts have several significant advantages over traditional cast or machined parts. P/M parts can be molded with very intricate features that eliminate much of the cutting that is required with conventional machining. P/M parts can be molded to tolerances within about 4 or 5 thousandths, a level of precision acceptable for many machine surfaces. Surfaces which require tighter tolerances can be quickly and easily machined since only a very small amount of metal need be removed. The surfaces of P/M parts are very smooth and offer an excellent finish which is suitable as a bearing surface.
The P/M process is also very efficient compared with other processes. P/M processes are capable of typically producing between 200-2000 pieces per hour depending on the size and the degree of complexity. The molds are typically capable of thousands of service hours before wearing out and requiring replacement. Since almost all of the powdered metal which enters the mold becomes part of the finished product, the P/M process is about 97% materials efficient. During sintering, it is only necessary to heat the green part to a temperature which permits fusion of the metal powder granules. This temperature is typically much lower than the melting point of the metal, and so sintering is considerably more energy efficient than a comparable casting process.
P/M parts are inherently somewhat porous. Due to the nature of the metal powder and the compaction process, there are inherently some voids where the metal powder particles are not completely compacted. These voids are a function of compaction pressures and powder particle geometry. Consequently, the voids (and hence the porosity) can be controlled to whatever degree desired. Structural parts can be produced that are 80-95% as dense as solid metal parts with comparable mechanical strengths.
The porosity of P/M parts can be exploited to advantage. The voids essentially represent a "cavernous" network that permeates the microstructure of a P/M part. These voids can be vacuum impregnated with oil to create self-lubricated parts with properties that cannot be matched by conventional cast and machined parts. The porosity also creates significant sound damping which results in quieter parts that do not vibrate or "ring" during operation. Also, the pores can be filled with corrosion-resisting materials or "infiltrated" with molten metals to provide various material and metallurgical properties that could not be attained in conventional cast and machined parts.
In spite of the many advantages of P/M parts, they have previously suffered from certain drawbacks. P/M parts are molded under high pressures which are attained through large opposing forces that are generated by the molding equipment. These forces are applied by mold elements which move back and forth in opposing vertical linear directions. The P/M parts produced thereby have previously necessarily had a "vertical" profile. Such conventional mold tooling and operation requirements do not allow the formation of transverse features which are indented or recessed between the ends of the molded part. An example of such a P/M element illustrating the vertical profile limitation is shown in FIG. 1. Also, P/M parts must necessarily have a vertical profile to facilitate their release from the mold. Since mold elements move back and forth in opposing vertical directions, P/M parts formed with transverse features, i.e. grooves, undercuts, crosscuts or threads would inhibit mold release. As seen in FIG. 2, such profile features had previously required a secondary machining step which adds greatly to the cost of the part, creating an economic disincentive to P/M fabrication.
The conventional P/M process is also not suitable for fashioning elements that have steeply sloped surfaces. If a surface is too steeply tapered the mold pressures will force the powder from the mold, thus prohibiting the formation of a tapered portion. Thus, tapered members of this type also require secondary machining.
Previous attempts have been made to provide P/M parts with other than a transverse profile. One such attempt is to use a split die. With this method a die is provided which has a transverse profile features incorporated onto the die surface. The die is vertically split into sections which reciprocate horizontally. After compaction by the vertical application of force, the split die opens horizontally to release the green part. This method is very limited. The transverse profile section cannot be too large or else it will interfere with powder fill. Also, a large profile could interfere with mold release, resulting in damaged green parts and equipment down time. Additionally, the transverse profile section cannot be too small or else the die section becomes prone to breakage under the compaction pressures. In general, the mechanics of split die compaction are very complicated and prone to difficulties. In view of the limitations and complications of this technique, split die compaction does not provide an economically viable alternative to the conventional P/M process.
Another method of creating P/M parts with grooves, undercuts and the like is to sinter bond two green parts. As seen in FIG. 3, two parts with appropriately tapered surfaces are individually compacted and fitted together prior to sintering. Upon sintering, the two parts become bonded together to form an integral part with an appropriately placed groove or undercut. While this method is effective, a double compacting step is required since each part must be formed separately and then assembled prior to sintering. The sinter bonding process also requires two complex sets of tools as well as careful material considerations. Thus, this technique also fails to provide an economically viable alternative to the conventional P/M process.